I've put together a DMX Shield with a protoboard and have been trying out the SerialtoDmx sketch. My issue is that when I start to send any sort of commands to the arduino I get really erratic behavior from the light I'm trying to control. My initial thought is that I'm picking up noise somewhere. Could all of the jumpers be acting as antennas and creating the erratic behavior?

I'm thinking this maybe more of a timing issue. I guess. What Arduino do you use? I made an Open DMX board using Arduino, first I use the old Diecimila board with ATMEGA 168 and it has some troubles, blinking erraticly to be exact. Then I switch to Duemilanove with ATMEGA 328 and the problem disappeared. From what I read, the ATMEGA 328-based Arduino board seems like the best option for DMX, not sure about the newer Arduinos

I did look into that library. The problem with that is that I have spent some time getting some software ready based on the DMX Simple library. I know that my process is probably backwards... Lesson learned. Is there anyway for me to track down the timing problems? If so where should I start?

I had some problems with a DMX circuit I made. I was using a sn75176 as the RS485 driver and couldn't get it to work reliably without a 0.1uf capacitor tying it's vcc and gnd pins. Have you tried testing the circuit in a breadboard? When I first built the circuit on a breadboard I didn't have a problem because of it's inherent capacitance.

Decoupling is not an optional thing, it is required for all digital logic. If you look at the datasheet for the MAX485CPA you'll find it mentions nothing about decoupling requirements despite the chipswitching highish currents at high speed - shame on Maxim! I'd suggest a 0.1uF ceramic capclose to the supply/ground pins as a reasonable first step. Close to the supply/ground pins isimportant.

I'd expect almost any capacitor of 10nF or higher to make a difference if that is the causeof the erratic behaviour if you want to check quickly, but you should always have decoupling on all chipsif you want circuits that are reliable.

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